The preparation of vinyl esters by reacting epoxy compounds with ethylenically unsaturated monocarboxylic acids, particularly in the presence of an esterification catalyst, e.g., an onium salt, is well known. See, for example, U.S. Pat. No. 3,377,406. Generally, such polyethylenically unsaturated polyesters (vinyl esters) are prepared by reacting a diglycidyl polyether of a dihydric phenol with two moles of an ethylenically unsaturated acid, such as acrylic or methacrylic acid, in the presence of an onium salt such as tetramethylammonium chloride. The resulting polyester has the general formula ##STR1## wherein each R is a hydrogen or an alkyl radical such as methyl and R.sub.1 is an aromatic radical, preferably a ##STR2## radical. Such vinyl esters are essentially full esters, i.e., free of epoxy groups, and may be blended with ethylenically unsaturated monomers such as styrene. The neat resin or the blend may be subsequently cured with peroxides to form products having excellent properties. However, these full esters sometimes exhibit viscosities which are too high, which limits their use. If, on the other hand partial esters are produced, the resulting partial esters exhibit lower viscosity but also exhibit poor shelf life. It will be appreciated that partial esters are produced by reacting the epoxy compound with less than the stoichiometric amount of the ethylenically unsaturated monocarboxylic acid, generally from about 50% to 85% of the stoichiometric amount of acid. It will be further appreciated that the resulting partial ester contains both epoxy and vinyl groups, or, more accurately, will be a blend of resins comprising resins having two terminal epoxy groups; two terminal vinyl groups; and one terminal epoxy group and one terminal vinyl group.
It has now been found that these partial vinyl esters (epoxyvinyl esters) can be stabilized and cured by a special dual path technique. Simply, excellent shelf life, even at 60.degree. C., and excellent curability can be achieved by such dual path approach wherein the curable composition is exposed to UV radiation followed by a post thermal treatment.